[llvm] [CMake] Make precompiled headers opt-in for ccache builds (PR #141927)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Also, has the issue been observed at all on the Github hosted runners? They use a different caching setup and I've only seen reports on the Buildkite side. None of the issues have links to Github workflows but that doesn't necessarily indicate anything.

https://github.com/llvm/llvm-project/pull/141927